North Korea is facing international condemnation after investigators blamed it for the sinking of a South Korean warship in March.
Pyongyang rejected the claim as a "fabrication" and threatened war if sanctions were imposed.
The international report found a North Korean submarine's torpedo sank the South Korean navy ship, causing the deaths of 46 sailors.
China urged restraint and did not criticise the North.
US Defence Secretary Robert Gates said Washington supported the findings of the investigation.
He added that the Pentagon was in "close consultation" with Seoul to determine the best way to respond.
Britain, Australia and Japan also expressed anger at North Korea. South Korean President Lee Myung-bak pledged to take "stern action".
UN Secretary General Ban Ki-moon said the report was "deeply troubling".
Britain's Foreign Secretary William Hague said North Korea's actions would deepen the international community's mistrust.
The investigation team, which included experts from America, Australia, Britain and Sweden, said it had discovered part of the torpedo on the sea floor and it carried lettering that matched a North Korean design.
Pyongyang said it would send its own inspection team to the South, to "verify material evidence" behind the accusation.
A North Korean defence spokesman said the country would "respond to reckless counter-measure with an all-out war of justice", the state KCNA news agency reported.
Chinese foreign ministry spokesman Ma Zhaoxu said Beijing had "noted" the report and would make its own assessment, but called on both sides to exercise restraint.
The Cheonan went down near the disputed inter-Korean maritime border, raising tension between the two nations, which technically remain at war.
The shattered wreck of the 1,200-tonne gunboat was later winched to the surface, in two pieces, for examination.
Investigators examined eyewitness accounts, damage to the vessel, evidence collected from the seabed and the injuries sustained by survivors and those who died.
There had earlier been a number of explanations suggested for the sinking, including an accidental collision with an unexploded sea mine left over from the Korean War.
Mr Lee's presidential office said he had told Australian Prime Minister Kevin Rudd his government would be taking firm measures against the North, and through international co-operation would make the North admit wrongdoing.
Japan's Prime Minister said in a statement that North Korea's action was "unforgivable".
Yukio Hatoyama said Japan would support South Korea if it sought a UN Security Council resolution against North Korea.
However the BBC's John Sudworth in Seoul says agreeing on an international response will be difficult as the diplomatic options will be limited.